This research will investigate: (a) subjective responses of normal humans to subanesthetic concentrations of nitrous oxide (N2O) in oxygen; and (b) the possible relationship between subjective N2O responses and several mediating variables, including subjects' antecedent drug abuse, selected personality traits, expectations of the drug experience (set), Psychomotor activity level during drug intoxication, drug concentration (dose), duration of inhalation, and physical setting during inhalation. Results should give a comprehensive picture of subjective N2O effects, may provide data and methods for comparative psychoactive drug studies, and may contribute to knowledge of the psychopharmacology of abused drugs more generally.